The harvesting or conversion of small-scale sources of mechanical energy into usable forms of electrical energy is an area which has attracted significant attention in recent years, and as a technology field has undergone rapid and substantial development.
One field in particular which has been the focus of much attention is that of triboelectric energy generation. The triboelectric effect (also known as triboelectric charging) is a contact-induced electrification in which a material becomes electrically charged after it is contacted with a different material through friction. Triboelectric generation is based on converting mechanical energy into electrical energy through methods which couple the triboelectric effect with electrostatic induction. It has been proposed to make use of triboelectric generation to power wearable devices such as sensors and smartphones by capturing the otherwise wasted mechanical energy from such sources as walking, random body motions, the wind blowing, vibration or ocean waves (see, for example: Wang, Sihong, Long Lin, and Zhong Lin Wang. “Triboelectric nanogenerators as self-powered active sensors.” Nano Energy 11 (2015): 436-462).
In its simplest form, a triboelectric generator uses two sheets of such dissimilar materials, one an electron donor, the other an electron acceptor. One or more of the materials can be an insulator. Other possible materials may include semiconductor materials, for example silicon comprising a native oxide layer. When the materials are brought into contact, electrons are exchanged from one material to the other, inducing a reciprocal charge on the two materials. This is the triboelectric effect.
If the sheets are then separated, each sheet holds an electrical charge (of differing polarity), isolated by the gap between them, and an electric potential is built up. If electrodes are disposed on to the two material surfaces and an electrical load connected between them, any further displacement of the sheets, either laterally or perpendicularly, will induce in response a current flow between the two electrodes. This is simply an example of electrostatic induction. As the distance between the respective charge centres of the two plates is increased, so the attractive electric field between the two, across the gap, weakens, resulting in an increased potential difference between the two outer electrodes, as electrical attraction of charge via the load begins to overcome the electrostatic attractive force across the gap.
In this way, triboelectric generators convert mechanical energy into electrical energy through a coupling between two main physical mechanisms: contact electrification (tribo-charging) and electrostatic induction.
By cyclically increasing and decreasing the mutual separation between the charge centres of the plates, so current can be induced to flow back and forth between the plates in response, thereby generating an alternating current across the load. Recently, an emerging material technology for power generation (energy harvesting) and power conversion has been developed which makes use of this effect, as disclosed in Wang, Z. L. “Triboelectric nanogenerators as new energy technology for self-powered systems and as active mechanical and chemical sensors.” ACS nano 7.11 (2013): 9533-9557. Based on this effect several device configurations have been developed of so-called triboelectric generators (“TEG”).
Since their first reporting in 2012, the output power density of TEGs has been greatly improved. The volume power density may reach more than 400 kilowatts per cubic metre, and an efficiency of ˜60% has been demonstrated (ibid.). In addition to high output performance, TEG technology carries numerous other advantages, such as low production cost, high reliability and robustness, and low environmental impact.
The TEG may be used as an electrical power generator, i.e. energy harvesting from, for example, vibration, wind, water, random body motions or even conversion of mechanically available power into electricity. The generated voltage is a power signal.
TEGs may broadly be divided into four main operational classes.
A first mode of operation is a vertical contact-separation mode, in which two or more plates are cyclically brought into or out of contact by an applied force. This may be used in shoes, for example, where the pressure exerted by a user as they step is utilised to bring the plates into contact. One example of such a device has been described in the article “Integrated Multilayered Triboelectric Nanogenerator for Harvesting Biomechanical Energy from Human Motions” of Peng Bai et. al. in ACS Nano 2013 7(4), pp 3713-3719. Here, the device comprises a multiple layer structure formed on a zig-zag shaped substrate. The device operates based on surface charge transfer due to contact electrification. When a pressure is applied to the structure, the zig-zag shape is compressed to create contact between the different layers, and the contact is released when the pressure is released. The energy harvested might be for example used for charging of mobile portable devices.
A second mode of operation is a linear sliding mode, wherein plates are induced to slide laterally with respect to one another in order to change the area of overlap between them. A potential difference is induced across the plates, having an instantaneous magnitude in proportion to the rate of change of the total overlapping area. By repeatedly bringing plates into and out of mutual overlap with one another, an alternating current may be established across a load connected between the plates.
One particular subset of linear sliding mode TEGs which have been developed are rotational disk TEGs which can be operated in both a contact (i.e., continuous tribocharging and electrostatic induction) or a non-contact mode (i.e., only electrostatic induction after initial contact electrification). Rotational disc TEGs typically consist of at least one rotor and one stator each formed as a set of spaced circle sectors (segments). The sectors overlap and then separate as the two discs rotate relative to each other. As described above, a current may be induced between two laterally sliding—oppositely charged—layers, with a magnitude in proportion to the rate of change of the area of overlap. As each consecutively spaced sector of the rotor comes into and then out of overlap with a given stator sector, so a current is induced between the two sector plates, initially in a first direction, as the plates increase in overlap, and then in the opposite direction as the plates decrease in overlap.
A design which enables energy to be harvested from sliding motions is disclosed in the article “Freestanding Triboelectric-Layer-Based Nanogenerators for Harvesting Energy from a Moving Object of Human Motion in Contact and Non-Contact Modes” in Adv. Mater. 2014, 26, 2818-2824. A freestanding movable layer slides between a pair of static electrodes. The movable layer may be arranged not to make contact with the static electrodes (i.e. at small spacing above the static electrodes) or it may make sliding contact.
A third mode of operation is a single electrode mode in which one surface is for example grounded—for example, a floor road—and a load is connected between this first surface and ground (see for example Yang, Ya, et al. “Single-electrode-based sliding triboelectric nanogenerator for self-powered displacement vector sensor system.”ACS nano 7.8 (2013): 7342-7351.) The second surface—not electrically connected to the first—is brought into contact with the first surface and tribocharges it. As the second surface is then moved away from the first, the excess charge in the first surface is driven to ground, providing a current across the load. Hence only a single electrode (on a single layer) is used in this mode of operation to provide an output current.
A fourth mode of operation is a freestanding triboelectric layer mode, which is designed for harvesting energy from an arbitrary moving object to which no electrical connections are made. This object may be a passing car, passing train, or a shoe, for example. (Again, see “Triboelectric nanogenerators as new energy technology for self-powered systems and as active mechanical and chemical sensors.” ACS nano 7.11 (2013): 9533-9557).
There are still further designs of triboelectric generator, such as a double-arch shaped configuration based on contact electrification. A pressure causes the arches to close to make contact between the arch layers, and the arches returns to the open shape when the pressure is released. A triboelectric nanogenerator has also been proposed which is formed as a harmonic resonator for capturing energy from ambient vibrations.
State of the art triboelectric nanogenerators, as for example presented by the Georgia Institute of Technology, are presently able to demonstrate only low power outputs in the range of a few milliwatts. In particular, the typical output power of a TEG currently consists of a voltage level in the range of a few hundreds of volts and a current level in the range of a few milliamps. In addition, the output of known TEGs consists of a high frequency regularly repeating pattern of high voltage pulses. This is a result of the periodic layout of electrodes in the known devices, in combination with a relatively high rate of motion.
Such high frequency, high voltage outputs are unsuitable as a direct power supply for many of the most common practical applications, and often require conversion by means of one or more transformer or amplifier circuits before they can be used in powering components. However, certain classes of devices do exist which can be directly driven by such outputs: in particular devices such as electroactive polymer (EAP) devices, LCDs, and electrophoretic devices such as displays or micro fluidic devices (especially those displaying dielectrophoretic behaviours).
Electroactive polymers in particular represent one of the most promising technologies for direct power and control by TEGs, since these materials may be used to form the basis of micro-scale actuator devices which offer the advantage of extremely low mechanical complexity, high reliability and cheap manufacturing costs. The input voltage requirements of EAPs are similar to those levels typically outputted by state of the art TEGs, making them particularly suitable for direct driving. However, while voltage and current levels are well matched, input and output frequencies are not, with TEG output frequencies typically falling in a range of 100 Hz-5000 Hz, but with EAPs more commonly requiring input frequencies closer to a range of 0.1 Hz-10 Hz (to make them suitable for practical applications such as actuation of skin for example).
Moreover, other varieties of generator (e.g. electret based) operating on similar principles, but not specifically utilising the triboelectric effect, also may suffer from this same drawback of providing high voltage outputs at frequencies unsuitable for direct driving of common components. Such generators might include in general any electrical power generator which operates through the relative motion of two or more charged elements, including for example induction-based generators which generate electrical power through electrostatic induction but which do not operate through tribo-charging of mutually moving elements.
One solution to the frequency mismatch of state of the art generators is simply to provide, in addition to the generator, a waveform modifier, such as a transformer or amplifier circuit, which is capable of modifying the output frequency of the generator. Such additional circuitry however, naturally adds power consumption, additional costs and complexity to the overall system and is hence undesirable from the point of view of efficiency, cost and simplicity.
Given the above described inadequacies in state of the art generators, there is a need for a system for generating directly from the relative motion of mutually charged generating elements an electrical output current having particular desired waveform, where said system does not require a dedicated waveform generator or waveform manipulator.